Reapply "[CodeGen][X86] Expand USUBSAT to UMAX+SUB, also for vectors"

Related to https://bugs.llvm.org/show_bug.cgi?id=40123.

Rather than scalarizing, expand a vector USUBSAT into UMAX+SUB,
which produces much better code for X86.

Reapplying with updated SLPVectorizer tests.

Differential Revision: https://reviews.llvm.org/D56636

llvm-svn: 351219